1. Field of the Invention
The present invention relates in general to sensing and keeping of freshness of vegetables and, more particularly, to a method for producing a vegetable freshness sensor or a gas sensor suitable for sensing gases generated by vegetables and sensing freshness of the vegetables and to a vegetable freshness keeping device using such a gas sensor suitable for automatically appropriately controlling the conditions of a vegetable store room in accordance with vegetable freshness sensing result of the gas sensor and keeping the freshness of vegetables for a long time.
2. Description of the Prior Art
Generally, freshness of food has been subjectively discriminated by the senses of the human being, such as the sense of sight, the sense of taste and the sense of smell, so that the discrimination of the food freshness using the senses may be indefinite. In an effort to overcome indefiniteness of discrimination of the food freshness using the senses, freshness sensing technique using a semiconductor gas sensor has been studied. In recent, there has been proposed a semiconductor gas sensor suitable for sensing trimethylamine (TMA) that is a typical component of odor generated by a fish.
With reference to FIG. 1, there is shown a typical semiconductor gas sensor suitable for sensing TMA. In this drawing, the reference numeral 1 is an insulating ceramic tube 1 which is coated with a metal electrode wire 2. The ceramic tube 1 in turn is coated with a sensing material 3 on the electrode wire 2. Axially extending through the ceramic tube 1 is a heater 4 for appropriately controlling the inner temperature of the tube 1. Representative example of the sensing material 3 suitable for sensing TMA is semiconducting oxide of titanium dioxide (TiO.sub.2). When using the titanium dioxide (TiO.sub.2) as the sensing material, the titanium dioxide (TiO.sub.2) is added with a metal catalyst so as to improve sensitivity of a resulting gas sensor. The basic factors influencing the sensitivity of the resulting gas sensor are catalytic activity and sensing film thickness. In order to improve the sensitivity of the gas sensor, it is very important to determine the quantity of used metal catalyst as well as the kind of used metal catalyst.
Abstract of prior researches of metal catalysts added to the titanium dioxide (TiO.sub.2) is shown in FIG. 2. FIG. 2 is a table showing expected effects and expected maximum sensitivities of resulting gas sensors in accordance with kinds of metal catalysts and quantities of metal catalysts added to the titanium dioxide (TiO.sub.2). The maximum sensitivities of the gas sensors are calculated through the equation R(air)/R(gas), wherein R(air) is a resistance of the sensor in air, while R(gas) is a resistance of the sensor in air laden with 300 ppm of TMA. As shown in the table of FIG. 2, it is noted that the metal catalysts such as ruthenium (Ru), indium (In) and gold (Ag) remarkably improve the sensitivity of the gas sensor for TMA.
It should be understood that there may exist gas sensors having other sensing materials besides titanium dioxide (TiO.sub.2) for sensing TMA. For example, In.sub.2 O.sub.3 --MgO (5 mol %) may be used as sensing material of the TMA gas sensor. That is, when adding 5 mol % of MgO is added to In.sub.2 O.sub.3, the electron concentration is reduced due to valency control, so that the sensor resistance in air is increased. When the sensor resistance in air reaches a given level, the sensitivity of the TMA gas sensor is improved.
However, the above researches are not practically used in industrial field but is still in an early stage. Furthermore, the gas sensor of FIG. 1 not only wastes electric power but also is not suitable for produced through an automated production process, so that the gas sensor has a problem that it can not be produced through mass production. Particularly, sensors for freshness of vegetables, which sensors are connected to this invention, should be particularly sensitive to gaseous sulfides generated by vegetables, so that such sensors should be produced through a specified process different from that of the above TMA gas sensors.